1. Field of the Invention
The invention relates to an image detection device comprising an image detection array which incorporates radiation sensitive elements to convert incident radiation into charges, and read-out-lines to transfer said charges to a read-out circuit which is arranged to convert transferred charges into a primary electronic image signal. The invention aim relates to an x-ray examination apparatus provided with an image detection array.
2. Description of the Related Art
An image detection array of said kind has been described in the European Patent Application EP 0 440 282 which corresponds to commonly-owned U.S. Pat. No. 5,184,018.
In the cited reference an image detection array is described which is in particular intended for use in an x-ray examination apparatus. X-ray examination is performed by irradiating a patient with an x-ray beam, so that an x-ray image is formed on an x-ray detector. The x-ray detector of the x-ray examination apparatus of the cited reference has the form of an image detection array. The x-ray image can either be converted into charges by the radiation sensitive semiconductor elements, or the x-radiation which is incident on the x-ray detector can be first converted by a scintillator layer e.g. a CsI:T1 layer into electromagnetic radiation of a longer wavelength, notably in the region between ultraviolet and infrared radiation. Next, the radiation produced by the scintillator layer is converted into charges by the semiconductor elements. The semiconductor elements may consist e.g. of .alpha.-Si:H, .alpha.-Se, or .alpha.-Se:As which are sensitive to x-radiation, or .alpha.-Si:H semiconductor elements covered by a CsI:T1 layer can be employed. The radiation sensitive elements of the array are arranged in rows and columns, so that the image detection array is shaped as an image detection matrix. The charges are read-out by way of read lines under control of a row-driver circuit which controls switching elements e.g. having the form of thin-film field effect transistors which are provided for each of the radiation sensitive elements. The read lines transfer the charges to a read-out circuit which forms a primary electronic image signal from the transferred charges. The primary electronic image signal is employed to display the image which is detected by the x-ray detector on a monitor, store the image information in an image memory or make a hard-copy of the image.
In the primary electronic image signal formed by an image detection array of the cited reference, artifacts due to memory effects can occur, adversely affecting the diagnostic quality of the stored and/or displayed images. Such artifacts notably have the form of phantom images and are particularly disturbing during fluoroscopy operation where a sequence of images is made at a low level of x-irradiation. The memory effects are caused by the fact that charges which are formed in the semiconductor material by the incident radiation become temporarily trapped in localised energy levels having an energy in the bandgap of the semiconductor material. The trapping of charges also causes a reduction of the amplitude of the primary electronic image signal. When trapped charges are thermally released with some time-delay they give rise to a phantom image which causes disturbances in a sequence of x-ray images because phantom images of an earlier image become superimposed on a next image in the sequence. Phantom-images occur especially just after one or a series of high-dose x-ray exposures have been made. A further cause of phantom images is the delay due to electrical capacitances of the radiation sensitive elements. Consequently, the transfer of charges to the read-out circuit is delayed and phantom images occur especially when the separation in time of images of a sequence of images becomes less than the delay time of the transfer of charges.